Development, analysis and control of thermal stimulation in automated skin photorejuvenation

Abstract

A novel robotic system for skin photo-rejuvenation is presented, which uniformly delivers laser thermal stimulation to a subject's facial skin tissue. The robotised treatment is performed by a customised end-tool attached to a 6 DOF manipulator, the end-tool is instrumented with a cosmetic laser generator, a depth sensor, and a thermal camera. To plan the optimal trajectory of the manipulator, our system computes the surface model of the face and segments it into seven regions, then automatically fills these regions with uniform laser irradiation. Finally, we report the detailed results of experiments on multiple human subjects to validate the performance of the system. To the best of the author's knowledge, this is the first time that facial skin rejuvenation has been automated by robot manipulators. This thesis also presents the development of a 3D physics-based numerical model of skin capable of representing the laser–skin photo-thermal interactions occurring in skin photorejuvenation treatment procedures. This model aimed to provide a rational and quantitative basis to control and predict temperature distribution within the layered structure of the skin. Ultimately, this mathematical and numerical modelling platform will guide the design of an automatic robotic controller to precisely regulate skin temperature at desired depths and for specific durations. The Pennes bioheat equation was used to account for heat transfer in a 3D multi-layer model of skin. The effects of blood perfusion, skin pigmentation and various convection conditions are also incorporated in the proposed model. The photo-thermal effect due to pulsed laser light on the skin is computed using light diffusion theory. The physics-based constitutive model was numerically implemented using a combination of finite volume and finite difference techniques. Direct sensitivity routines were also implemented to assess the influence of constitutive parameters on temperature. A stability analysis of the numerical model was conducted. Finally, the numerical model was exploited to assess its ability to predict temperature distribution and thermal damage via a multi-parametric study which accounted for a wide array of biophysical parameters such as light coefficients of absorption for individual skin layers and melanin levels (correlated with ethnicity). It was shown how critical is the link between melanin content, laser light characteristics and potential thermal damage to the skin. The developed photo-thermal model of skin-laser interactions paves the way for the design of an automated simulation-driven photorejuvenation robot, thus alleviating the need for inconsistent and error-prone human operators. Lastly, a thermal dose controller (TDC) for thermally stimulating the skin surface in photorejuvenation procedures was designed, modelled and evaluated. The TDC includes the ability to precisely control temperature profiles. A mathematical model of the TDC, herein referred as a predictive model controller (PMC), was developed and assessed on a three-dimensional biophysics-based numerical model of skin while its hardware implementation was physically tested on a gelatin-based phantom tissue subjected to pulsed laser irradiation. Owing to the lack of consensus on the choice of appropriate thermal dose metrics used by the photo-dermatology community we proposed a new thermal dose unit. A modified thermal dose unit is proposed in order to monitor and control the thermal dose during treatment. The robotic laser system and its TDC were evaluated for various laser pulse durations on both the numerical model of skin and the tissue phantom. Results demonstrated that the developed TDC endowed with the proposed dose unit could precisely deliver the prescribed laser irradiation and thermal dose on the tissue surface.